As the easy-to-access and easy-to-produce hydrocarbon resources depleted over the last century, more and more difficult wells remain. Moreover, as the world's hydrocarbon demand is continuously growing, meeting this demand requires development of more advanced recovery procedures. One such procedure is Steam Assisted Gravity Drainage (“SAGD”), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from an injector wellbore that is drilled above and parallel to another producer wellbore.
As the viscosity of the heavy oil in the formation around the injector wellbore is reduced, the heavy oil drains into the lower producer wellbore, from which the oil is extracted. Preferably, the two wellbores are drilled at a distance of only a few meters from one other. The placement of the injector wellbore needs to be achieved with very small margin in distance.
Therefore, such downhole procedures present some challenges. If the injector wellbore is positioned too close to the producer wellbore, the producing well would be exposed to very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the SAGD process is reduced. It is well known that traditional magnetic and gravity-based surveying techniques suffer from a widening cone of uncertainty as the wells become longer, making it more difficult to achieve the precision in placement that is required in SAGD applications.
Existing acoustic ranging methods depend on time of flight calculations that assume a velocity profile that may not be accurate due to measurement errors, changes in the well path, and horizontal variations in acoustic properties of formations. Such measurements are further complicated by the effect of boreholes on the time of flight calculation, which adds uncertainty to the time of flight estimation.
Accordingly, there is a need in the art for an improved downhole acoustic ranging techniques.